The Real Way of Comparing Engine Designs

A better means of comparison

I was very blown away by the new 6.3 litre,
375kW AMG engine recently featured in AutoSpeed - until I thought about it a bit
more...power per litre. 375kW from a 6.3 litres equals 59.53kW per litre....not
bad really. But my 5 year old Honda 1.8 litre Type R produces 141kW in standard
form = 78.3kw per litre (standard) and now with 151kW (ECU mod.) = 83.88kW per
litre...

But the problem is that comparing the specific
power of an engine – that is, how many kilowatts per litre – actually tells you
very little about how good an engineering job has been done. Why? Well, it takes
into account only two aspects of the engine – how big it is, and how much power
it develops. But since for a given torque value, power is dependent on engine
revs, without including engine rpm in the comparison it simply has little
significance.

So how do you do it, then?

BMEP

The answer is to calculate the engine’s Brake Mean
Effective Pressure, usually abbreviated to BMEP.

Let’s take it step by step.

‘Brake’ in this context is the same as used when
saying ‘brake horsepower’ (bhp). The brake refers to an engine brake that is
applied to the crankshaft output. (We normally call an engine brake a dyno.) So
whenever the word ‘brake’ is used, it refers to an actual engine output, as
opposed to a theoretically calculated figure that might not take into account
internal frictional losses, etc. ‘Mean’ is a fancy way of saying average, as in
the mathematically calculated average value. ‘Effective Pressure’ is just as the
name suggests – the pressure in the combustion chamber that actually has an
effect on the piston.

So, BMEP refers to the average pressure that acts
on the piston during the engine’s four strokes. The higher it is, the more the
design has been optimised. And the key thing about BMEP is that it takes into
account engine rpm, engine volume and engine power output. It’s the only
equation to use when comparing engines from the perspective of saying which is
more highly developed. It doesn’t matter whether the engine uses pushrods; 2
valves per cylinder, 4 valves per cylinder or 5 valves per cylinder. It doesn’t
matter whether the engine revs to 8000 rpm or 1500 rpm. You can use the equation
to directly compare any 4-stroke engine, whether it’s 660cc or 5.7 litres.

And you can even use it to compare naturally
aspirated and forced induction engines. The BMEP figure for a forced induction
engine will have a major advantage, but it’s still a valid method of
comparison.

Calculating BMEP

OK, so how do you use this wondrous formula?
Keeping it all metric, it looks like this:

kW x 1200
BMEP = ---------------
litres x rpm

This gives BMEP in Bar, but that doesn’t really
matter that much because it’s the comparison number we’re after, not the
absolute.

So let’s take a look at some engines. The Honda
Type R engine mentioned by our reader in the first paragraph (or the one we
tested in 2002, anyway) has a 2-litre engine that develops 147kW at 7400 rpm.
Put the figures into the equation and a BMEP of 11.9 Bar results.

Now, what about the AMG 6.3 litre? It has 375kW at
6800 rpm, giving a BMEP of 10.5 Bar. So yep, the Honda engine does
better.

But before Honda owners get too cocky, how about
some other comparisons?

The BA series Australian Ford Falcon six cylinder
uses a 4-litre straight six to develop 182kW at 5000 rpm – and that’s a BMEP of
10.9 Bar! OK, it’s not as high as the Honda engine but it’s better than
the AMG!

(Confused? How can the humble Falc engine do
better than the AMG? The other way to look at it is that the Falcon engine
develops a fair amount of power for its four litre capacity – and it does so
without revving very hard at all. Since power = torque x rpm, the Falcon engine
is developing a lot of torque – and to do that, it needs to breathe very well.
And breathing well will result in a high Mean Effective Pressure.)

So what about some other engines?

The Hyundai Sonata V6 is one of the more
impressive engines we’ve driven lately. It develops 173kW at 6000 rpm from its
3.3 litre engine. You can see just by glancing at the figures that it’s going to
be close, but probably a bit worse, than the Falcon. How come? Well, the
Sonata’s power is down a bit, the engine a little smaller – but it needs higher
revs to do it. And when you do the equation you’ll see the Sonata V6 has a BMEP
of 10.5 Bar.

The latest iteration of the Holden Monaro sees it
equipped with a 6-litre V8 developing 260kW at 5600 rpm. BMEP? It’s a paltry
9.3 Bar!

The Peugeot 206 GTi 180? It uses a 2-litre engine
to produce 130kW at 7000 rpm. Will this challenge the Honda? Nope, not with
11.1 Bar although that high figure shows the degree of engine
development.

In
his 2001 book, Formula 1 Technology, Peter Wright lists the BMEP of the
1967 Ford-Cosworth DFV as 14 Bar at 8500 rpm and a 2001 F1 engine at
13.8 Bar – but that’s at 17,500 rpm.

One of the most underrated engines we’ve ever
driven is that in the 180kW Ralliart Magna. From its 3.5 litre V6 it develops
180kW at just 5500 rpm. So, lots of power, a smallish engine and the max power
output achieved at low revs. That combination should show a high BMEP! And at
11.2 Bar, it’s the closest yet to the Honda Type R. (And achieved without
a variable inlet manifold and variable cam timing, and with a huge spread of
on-road torque!)

But there’s no getting away from it, Honda does
produce excellent naturally aspirated engines, whatever the method of analysis.
Take the 3-litre Honda Accord V6, with 177kW at 6250 rpm. Do the figures and
you’ll see a BMEP of 11.3 Bar!

Of course, to get sky-high BMEP figures you can
force more gas into the combustion chamber, using either a turbo or
supercharger. We saw how good the BMEP was of the naturally aspirated Falcon,
and the XR6 turbo (with 240kW at a very low 5250 rpm from its 4 litres) has a
BMEP of 13.7 Bar. (Some of the modified ones must have huge BMEPs!)

The SR20DET-powered Nissan 200SX in its last S15
model developed 147kW at 6400 from its 2 litres. Just looking at those three
figures you might figure that BMEP isn’t going to be very high – but at 13.8
Bar it’s better than the Falcon.

But then take a look at the Mini Cooper S with its
supercharged 1.6, developing 120kW at just 6000 rpm. It has a BMEP of 15
Bar!

Conclusion

BMEP figures don’t tell you the whole story. They
don’t show BSFC (Brake Specific Fuel Consumption) and they don’t show the spread
of engine torque, the engine’s flexibility or any of those other real world
characteristics. But a BMEP figure is still a far better means of comparison
that kW/litre... even if calculating BMEP shows just how good that Honda Type R
engine really is!

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